Processing apparatus and method for processing metal film

ABSTRACT

A method for processing a metal film includes adiabatically expanding a mixed gas including an oxidation gas, a complexing gas and a rare gas in a processing chamber having a vacuum exhaust device such that a gas cluster beam is generated in the processing chamber, and irradiating the gas cluster beam upon a metal film formed on a surface of a workpiece in the processing chamber such that the gas cluster beam collides on the metal film including a metal element and the metal film is etched. The mixed gas includes the oxidation gas which oxidizes the metal element and forms an oxide, and the complexing gas which reacts with the oxide and forms an organometallic complex

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of PCT International Application No. PCT/JP2012/050869, filed Jan. 17, 2012, which is based upon and claims the benefit of priority to Japanese Patent Application No. 2011-013313, filed Jan. 25, 2011. The entire contents of these applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a processing apparatus and to a method for processing metal film formed on a surface of a workpiece such as a semiconductor wafer by using gas cluster beams.

2. Description of Background Art

Nowadays, wiring of super LSIs is formed using a copper damascene process. Copper damascene processing is such a method as follows: First, patterned grooves are formed in insulative film using photolithographic and dry-etching techniques, the surface of the film is covered with copper barrier film and copper plating is deposited in the grooves. Then, unwanted upper-layer portions are removed by CMP (chemical mechanical polishing) so that metal patterns are formed (see D. Edelstein et al., IEDM Technical Digest, IEEE (1997)).

Another process known for forming metal film patterns other than a damascene process is wet etching. In such a method, a mask is patterned on metal film, and portions without the mask are processed by wet etching using dilute hydrochloric acid or the like.

Yet another process is an RIE (reactive ion etching) method. In such a method, metal portions without a mask are etched by reactive plasma (see Y. Yasuda, Thin Solid Films, Volume 90, Issue 3, 23 April 1982, pages 259-270).

Halides of metals such as Co, Ni, Cu, Pt and Ru have low vapor pressures. When such a metal is etched by an RIE method for forming patterns, temperatures of the substrate and the reaction chamber are set high so that the metal halide is gasified and removed, and the metal halide is prevented from attaching to the walls of the reaction chamber. Using such a high-temperature RIE process, halogen ions and radicals, which are active species of plasma, corrode the side walls of openings (grooves and holes) formed by etching. Thus, it is difficult to retain excellent pattern forms (see B. J. Howard and C. Steinbruchel, Applied Physics letters, 59(8), 19p914 (1991)).

In an RIE method such as above, the etchant decomposed by the plasma remains as residue in a form of polymers or compounds, causing frequent problems such as difficulty in removing the residue even when wet cleaning is performed after the etching process. The entire contents of these publications are incorporated herein by reference.

SUMMARY OF THE INVENTION

According tone aspect of the present invention, a method for processing a metal film includes adiabatically expanding a mixed gas including an oxidation gas, a complexing gas and a rare gas in a processing chamber having a vacuum exhaust device such that a gas cluster beam is generated in the processing chamber, and irradiating the gas cluster beam upon a metal film formed on a surface of a workpiece in the processing chamber such that the gas cluster beam collides on the metal film including a metal element and the metal film is etched. The mixed gas includes the oxidation gas which oxidizes the metal element and forms an oxide, and the complexing gas which reacts with the oxide and forms an organometallic complex.

According to another aspect of the present invention, a processing apparatus for etching a metal film includes a processing chamber having a vacuum exhaust device, a holding device which holds a workpiece, a gas cluster beam generating device which is positioned to face the holding device and generates a gas cluster beam through adiabatic expansion of a mixed gas including an oxidation gas which oxidizes a metal element and forms an oxide, a complexing gas which reacts with the oxide and forms an organometallic complex, and a rare gas, and a gas cluster beam irradiation device which irradiates the gas cluster beam onto a metal film including the metal element formed on a surface of the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a diagram showing an example of a metal film processing apparatus according to an embodiment of the present invention;

FIG. 2A is a view of a step showing an example of a metal film processing method according to an embodiment of the present invention;

FIG. 2B is a view of a step showing an example of a metal film processing method according to an embodiment of the present invention;

FIG. 2C is a view of a step showing an example of a metal film processing method according to an embodiment of the present invention;

FIG. 3 is a graph showing a vapor pressure curve of Cu(hfac)₂, a reaction byproduct formed when copper is etched; and

FIG. 4 is a diagram showing a modified example of a processing apparatus according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings.

FIG. 1 is a diagram showing an example of a metal film processing apparatus according to an embodiment of the present invention. In the present embodiment, an etching process for forming patterns on a metal film is described using thin copper film.

As shown in FIG. 1, processing apparatus 2 has processing chamber 4 formed in a boxlike shape having a predetermined length. Processing chamber 4 is formed with material having excellent pressure resistance such as aluminum, aluminum alloy or stainless steel. Processing chamber 4 is divided into right and left sections by skimming plate 10 provided in the center of the chamber: one is processing space 6 in which to place a workpiece such as semiconductor wafer (W); and the other is a beam generating space in which to generate later-described gas cluster beams for processing. In the center portion of skimming plate 10, gas skimming aperture 12 is formed to exclusively pass straight forward-directed gas cluster beams.

Despite the miniature size of the opening area of gas skimming aperture 12, processing space 6 and beam generating space 8 are connected to each other by gas skimming aperture 12. In addition, holding device 14 to hold semiconductor wafer (W) is provided in processing space 6. More specifically, holding device 14 includes holding table 16 shaped like a disc, for example, to hold wafer (W). Holding table 16 is set vertically and the back surface of wafer (W) is attached to a side surface and the periphery of wafer (W) is secured by clamper 18. Holding table 16 is supported by scanning actuator 20 provided on the ceiling of processing chamber 4.

In particular, scanning actuator 20 has arm 22 that extends downward, and holding table 16 is securely attached to arm 22. Arm 22 is set to be movable vertically (directions Y) and horizontally (directions Z) as viewed in the drawing as well as in a direction perpendicular to the drawing sheet (directions X, not shown). In directions X and Y, arm 22 makes scanning movements to cover at least the length of a radius of wafer (W). According to such scanning movements, gas cluster beams coming in a straight-forward direction from the left side of the drawing are irradiated on the entire surface of wafer (W).

On the bottom portions of processing space 6 and beam generating space 8 divided in processing chamber 4, exhaust outlets (24, 26) are formed respectively, and vacuum exhaust device 28 is connected to exhaust outlets (24, 26). Exhaust device 28 is equipped with exhaust passage 30 connected commonly to both exhaust outlets (24, 26). In exhaust passage 30, pressure adjustment valve 32, first vacuum pump 34 and second vacuum pump 36 are provided in that order so that pressure is adjusted from the upstream side toward the downstream side. Accordingly, the pressure of entire processing chamber 4 is adjusted to maintain a high vacuum state. A turbomolecular pump, for example, is used for first vacuum pump 34, and a dry pump, for example, is used for second vacuum pump 36.

In processing chamber 4, gas cluster beam generating device 38 is provided to face holding table 16. More specifically, gas cluster beam generating device 38 has irradiation mechanism 40 to emit gas cluster beams at high speed. Irradiation mechanism 40 has long horizontal-shaped retainer chamber 42 with a predetermined capacity, and nozzle section 44 which is provided at the front-edge side of long horizontal retainer chamber 42 and shaped like a trumpet with a diameter gradually increasing in the irradiation direction. Irradiation mechanism 40 as a whole is shaped like a de Laval nozzle, for example.

Gas inlet passage 46, to introduce various gases for forming gas cluster beams, is connected to retainer chamber 42. Oxidation gas passage 48 to flow oxidation gas and complexing gas passage 50 to flow complexing gas are both connected to gas inlet passage 46. In oxidation gas passage 48, flow controller 52, such as a gas mass flow controller, and shutoff valve 54 are provided in that order from the upstream side toward the downstream side so that oxidation gas is supplied under high pressure while its flow rate is controlled. Here, an example of oxidation gas is O₂ (oxygen).

In the present embodiment, a complexing agent which is liquid at room temperature is used. Thus, in complexing gas passage 50, flow controller 56 such as a liquid mass flow controller, shutoff valve 58 and vaporizer 60 are provided in that order from the upstream side toward the downstream side. In addition, rare gas passage 62 to flow a rare gas as a carrier gas is connected to vaporizer 60. In rare gas passage 62, flow controller 64, such as a gas mass flow controller, and shutoff valve 66 are provided in that order from the upstream side toward the downstream side so that a rare gas is supplied as a carrier gas under high pressure while its flow rate is controlled.

Here, as for the above complexing agent, hexafluoroacetylacetone (1,1,1,5,5,5-hexafluoro-2,4-pentanedione: H(hfac)), which is liquid at room temperature, is used, and for the rare gas as a carrier gas, argon is used. The liquid complexing agent is flowed by being compressed at high pressure, vaporized in vaporizer 60 to become a complexing gas, mixed with a high-pressure carrier gas (Ar) and flowed downstream.

The oxidation gas, complexing gas and rare gas (carrier gas) are mixed and introduced from gas inlet passage 46 to retainer chamber 42 while being kept under high pressure. Then, gas cluster beams 70 are generated when the gas mixture adiabatically expands going through nozzle section 44 toward beam generating space 8 set to be in vacuum. At that time, since irradiation mechanism 40 is set in such a way that nozzle section 44 is positioned at the same height as the center of the opening of gas skimming aperture 12, the center of emitted gas cluster beams 70 passes through gas skimming aperture 12. Here, the oxidation gas works to form an oxide by oxidizing the element of a metal film formed on the surface of semiconductor wafer (W). Also, the complexing gas works to form an organometallic complex by a reaction with the oxide. The rare gas works as nuclei when gas clusters are formed.

Here, the oxidation gas and complexing gas (including carrier gas) were mixed in a passage. However, that is not the only option, and gases may be introduced separately to retainer chamber 42 and mixed in retainer chamber 42. Alternatively, when a carrier rare gas is not necessary to be added into a complexing gas, it is an option to form a mixed gas by mixing a rare gas with the oxidation gas or complexing gas in the passage, or to introduce a rare gas directly to retainer chamber 42 to form a mixed gas.

The entire operation of processing apparatus 2 structured as above is controlled by apparatus control device 72 containing computers or the like. Computer programs for the operation are stored in memory medium 74. Memory medium 74 is made of a flexible disc, CD (compact disc), hard disc, flash memory, DVD or the like, for example. In particular, supply initiation or interruption of various gases and adjustment of their flow rates, control of processing pressures, and so forth are conducted by commands from apparatus control device 72.

Apparatus control device 72 also has a user interface (not shown) which connects apparatus control device 72 and a device operated by an operator. The user interface may be a keyboard on which an operator enters input/output commands to control the apparatus, or may be a display that visually shows the operational status of the apparatus. Moreover, it is an option for apparatus control device 72 to be connected to a communication line (not shown) so that information for each control is sent to apparatus control device 72 through the communication line.

Processing Method

By referring to FIG. 1˜FIG. 3, following is a description of a metal film processing method according to an embodiment of the present invention, using processing apparatus 2 structured as above. FIG. 2A˜FIG. 2C are views of steps showing an example of a metal film processing method according to the present embodiment, and FIG. 3 is a graph showing the vapor pressure curve of Cu(hfac)₂ formed as a reaction byproduct when copper is etched.

First, semiconductor wafer (W) as a workpiece is held on holding table 16 of holding device 14 provided in processing chamber 4 by securing semiconductor wafer (W) with clamper 18. At that time, wafer (W) is positioned in such a way that its processing surface faces the left of the drawing to be opposite gas cluster beam generating device 38. Referring to FIG. 2A˜FIG. 2C showing the enlarged surface of wafer (W), metal film 72 to be etched is formed in advance on the processing surface of wafer (W) as shown in FIG. 2A, and patterned mask 74 is formed on the surface of metal film 72. Here, as described above, copper (Cu) is used as the metal film, and a material resistant to gas cluster beams is used for mask 74, for example, silicon oxide film (SiO₂) or silicon nitride film (Si₃N₄) formed by plasma CVD (chemical vapor deposition).

After wafer (W) is held on holding table 16 as above, processing chamber 4 is sealed, while exhaust apparatus 28 is driven to vacuum out processing chamber 4 so that processing space 6 and beam generating space 8 are set to be at high vacuum.

Gas cluster beam generating device 38 is driven to generate gas cluster beams 70. Namely, the high-pressure oxidation gas, complexing gas and rare gas are each flowed to be supplied while their respective flow rates are controlled. Since the complexing agent as the material for complexing gas, namely H(hfac), is liquid at room temperature, it is compressed at high pressure while its flow rate is controlled, and vaporized in vaporizer 60 to make it a complexing gas. The complexing gas is flowed out after being mixed with Ar gas (rare gas) as a carrier gas supplied to vaporizer 60. Then, the oxidation gas, complexing gas and rare gas form a mixed gas, which then passes through gas inlet passage 46 to be supplied to retainer chamber 42 of irradiation mechanism 40. The mixed gas is highly pressured, and emitted or emitted from nozzle section 44 while expanding adiabatically toward highly vacuumed beam generating space 8. At that time, since processing chamber 4 is at high vacuum, the mixed gas adiabatically expands when it is emitted, forming gas cluster beams 70, which are then irradiated on wafer (W).

Regarding gas cluster beams 70, scattered gas clusters are blocked by skimming plate 10, and only straight forward-directed gas cluster beams 70 pass gas skimming aperture 12 provided in skimming plate 10, and are irradiated on wafer (W) as shown in FIG. 2B. The pressure in retainer chamber 42 is approximately 20 atm, for example. The pressures in beam generating space 8 and processing space 6 are set at 10³ Pa or greater but 105 Pa or lower.

When the mixed gas adiabatically expands at nozzle section 44 and is cooled, atoms or molecules of the oxidation gas are loosely bonded with atoms or molecules of the complexing gas using rare gas Ar as the nuclei in gas clusters (70A) to form gas cluster beams 70. Namely, gas cluster (70A) is made up of a few to a few thousand atoms or molecules, for example, where oxidation gas, complexing gas and rare gas are mixed at the atom level or molecule level.

When gas cluster beam 70 is irradiated at Cu metal film 72 as shown in FIG. 2B, heat is partially generated from the impact energy. At that time, first, copper reacts with the oxidation gas to form an oxide, and the oxide reacts with the complexing gas to form an organometallic complex having a relatively high vapor pressure. When the organometallic complex is gasified and exhausted, Cu metal film 72 is etched as shown in FIG. 2B and FIG. 2C. Accordingly, metal film 72 exposed in groove 74 of patterned mask 74 is etched away by gas cluster beam 70.

Using scanning actuator 20, holding table 16 is scanned in directions X and Y so that the entire surface of wafer (W) is irradiated and etched by gas cluster beams 70. Also, by moving holding table 16 in directions Z, wafer (W) is set to be positioned closer to or away from irradiation mechanism 40 during the etching process. The reactions of copper with oxidation gas (O₂) and with complexing gas H(hfac) during the etching process are shown below.

4Cu+O₂→2Cu₂O (Cu: monovalent)

2Cu+O₂→2CuO (Cu: divalent)

Cu₂O+2H(hfac)→Cu+Cu(hfac)₂↑+H₂O↑

CuO+2H(hfac)→Cu(hfac)₂↑+H₂O↑

Arrows (↑) indicate scattering gas. Since organometallic complex Cu(hfac)₂ formed as a reaction byproduct has a relatively high vapor pressure, it is easy to remove it through sublimation. Here, unoxidized copper does not react with H(hfac) unless the temperature is at least 265° C. However, as described above, monovalent or divalent oxidized copper easily reacts with H(hfac) at an approximate temperature of 150° C. Therefore, the temperature of copper can be easily raised locally to 150° C. or higher from the impact energy of gas cluster beam 70, forming complex (organometallic complex) Cu(hfac)₂, which has a relatively high vapor pressure, namely, which is easily sublimated.

Since the complex has a relatively high vapor pressure as described above, it is easily sublimated and removed without heating wafer (W) itself to a higher temperature. FIG. 3 shows a vapor pressure curve of complex Cu(hfac)₂. For example, the vapor pressure is about 100 Torr at an approximate temperature of 150° C. Therefore, since the impact energy of gas cluster beam 70 is converted to thermal energy and easily reaches an approximate temperature of 150° C. microscopically, and the processing pressure in processing space 6 is lower than 100 Torr, for example, it is found that Cu(hfac)₂ is easily sublimated and removed.

As described above, when gas cluster beam 70 first collides on Cu metal film 72, most oxidization gas O₂ is consumed by the oxidation reaction with Cu on the collision surface. Thus, oxygen is hardly contained in the molecules scattered around from the collision surface. Even if unreacted oxygen molecules exist, they have lost kinetic energy from the secondary scattering, thus oxidation reactions are less likely to occur when they collide on side walls. Therefore, secondary etching seldom occurs from scattered molecules, and side etching is suppressed. Accordingly, the smoothness of the side walls of an etched groove of a metal film is retained.

In such a case, especially when the amount of oxidation gas to oxidize copper is set smaller than the amount of complexing gas, no excess oxidation gas remains, and thus above-described side etching is further suppressed. To sufficiently achieve the effects of suppressing side etching, the amount of complexing gas is preferred to be set at least 5 times as much as the amount of oxidation gas. Especially, by controlling the ratio of the amount of oxidation gas to the amount of complexing gas, the degree of side etching is adjusted and suppressed. In addition, a relatively small amount is required for rare gas. For example, 1/10 of the amount of oxidation gas is sufficient. However, the amount of rare gas is not limited specifically.

According to the present embodiment, a mixed gas is prepared by an oxidation gas such as O₂ to form an oxide by oxidizing the element of metal film 72 made of copper, for example, a complexing gas such as H(hfac) to form an organometallic complex by a reaction with the oxide above, and a rare gas, and gas cluster beams 70 are formed through adiabatic expansion of the mixed gas. Then, the gas cluster beams collide on the metal film of a workpiece such as semiconductor wafer (W) to etch the metal film as described above. Therefore, metal films which were unable to be etched by conventional cluster beam methods are etched using the cluster beams of an oxidation gas, complexing gas and rare gas according to the present embodiment.

Modified Example

Next, a processing apparatus according to a modified example of the embodiment is described. In the embodiment above, gas cluster beams 70 generated by gas cluster beam generating device 38 directly collide on semiconductor wafer (W). However, that is not the only option. For example, gas cluster beams 70 may be ionized while being accelerated and collide on the wafer. Such a modified example of a processing apparatus is shown in FIG. 4.

FIG. 4 is a diagram showing an example of a processing apparatus according to a modified example of the present invention. In FIG. 4, for a portion corresponding or identical to that shown in FIG. 1, the same reference number is applied and its description is omitted here. As shown in FIG. 4, on a side of processing space 6 in processing chamber 4, partition 80 is provided to be parallel to skimming plate 10, and ionization space 82 is formed between partition 80 and skimming plate 10.

In the center of partition 80, small-diameter irradiation aperture 84 is formed to be positioned in a straight line extending from gas skimming aperture 12 of skimming plate 10 and nozzle section 44 of irradiation mechanism 40. Gas cluster beams 70 are set to pass through the ionization space. In addition, at the bottom of processing chamber 4 divided into ionization space 82, exhaust outlet 86 is also formed, which is connected to exhaust passage 30 of exhaust apparatus 28. Thus, ionization space 82 is set to be in vacuum.

In ionization space 82, ionizer 88 is provided corresponding to the route where gas cluster beams 70 pass so that gas cluster beams 70 are ionized when passing through ionizer 88. An example of ionizer 88 is an electron impact ionizer with an incandescent filament (not shown) to emit thermally charged electrons for ionization.

On the downstream-side route of ionizer 88, electrode acceleration section 90 is formed to accelerate ionized gas cluster beams 70. Electrode acceleration section 90 has multiple pairs of ring-shaped electrodes 92 provided parallel to the progression direction of gas cluster beams 70. Then, an acceleration power source (not shown) is connected between the multiple pairs of electrodes 92 to apply high voltage for beam acceleration.

According to such a modified example, in addition to exhibiting the same effects as in the embodiment above, gas cluster beams 70 ionized by ionizer 88 are accelerated by electrode acceleration section 90 and then collide on wafer (W). Accordingly, etching is more efficiently performed on the metal film.

As described so far, according to methods for processing metal film and processing apparatuses of the embodiment and its modified example, excellent effects are exhibited as follows.

Using non-plasma processing, corrosion caused by halogen radicals or ions does not occur on side-wall surfaces. Thus, compared with RIE methods, excellent etching results are obtained. Also, etching gas is prevented from becoming active by plasma so that the gas is not polymerized to be deposited on mask surfaces or etching side walls. Accordingly, steps for cleaning the substrates after etching are significantly simplified.

Since an organometallic complex with a high vapor pressure is formed from the introduced gases and then exhausted, the amount of byproducts attaching to side walls of the processing chamber is drastically reduced. Therefore, the number of cleaning treatments on the inner walls of the processing apparatus is reduced, and the apparatus is used highly efficiently.

When processing a metal film on the surface of a workpiece, a mixed gas is prepared using an oxidation gas to form an oxide by oxidizing the element of metal film, a complexing gas to form an organometallic complex by a reaction with the oxide, and a rare gas, and the mixed gas is adiabatically expanded to generate gas cluster beams. Then, the gas cluster beams collide on the metal film of the workpiece to etch the metal film. Therefore, metal films which were unable to be etched by conventional cluster beam methods are etched using the cluster beams of an oxidation gas, complexing gas and rare gas according to the present embodiment.

In the embodiments above, examples were described in which 0 ₂ gas was used as an oxidation gas. However, that is not the only option, and one or more gases selected from a group of O₂, H₂O and H₂O₂ may be used as an oxidation gas.

In the embodiments above, examples were described in which H(hfac), an organic acid, was used as a complexing gas (complexing agent). However, that is not the only option, and one or more materials selected from among acetylacetone, hexafluoroacetylacetone (1,1,1,5,5,5-hexafluoro-2,4-pentanedione: H(hfac)), trifluoroacetic acid (TFA), formic acid, acetic acid, propionic acid, butyric acid and valeric acid may also be used as a source of complexing gas.

In the embodiments above, examples were described in which Ar was used as a rare gas. However, that is not the only option, and other rare gases such as He, Ne, Kr, Xe and the like may also be used. Also, in the above embodiments, examples were described in which copper as metal film 72 was etched. However, that is not the only option, and the present invention is also applied to an example in which one material selected from among Cu, Co, Ni, Pt and Ru is used as metal film 72 to be etched.

In the embodiments above, examples were described in which a semiconductor wafer was processed as a workpiece. However, the semiconductor wafer in the embodiments also includes compound semiconductor substrates such as a silicon substrate, GaAs, Sic, GaN and the like. Alternatively, the present invention is not limited to such substrates, and is also applied to glass substrates and ceramic substrates used in liquid-crystal display apparatuses.

A method for processing a metal film according to an embodiment of the present invention includes the following steps: in a processing chamber having a vacuum exhaust apparatus, a step for producing gas cluster beams through adiabatic expansion of a mixed gas containing an oxidation gas to form an oxide by oxidizing the element of a metal film, a complexing gas to form an organometallic complex by a reaction with the oxide, and a rare gas; and a step for etching a metal film by colliding the gas cluster beams on the metal film formed on a surface of a workpiece.

According to another embodiment of the present invention, a processing apparatus is provided for processing a metal film on a surface of a workpiece by using gas cluster beams. Such an apparatus includes a processing chamber having a vacuum exhaust apparatus; a holding device to hold the workpiece; and positioned opposite the holding device in the processing chamber, a gas cluster beam generating device to generate gas cluster beams through adiabatic expansion of a mixed gas containing an oxidation gas to form an oxide by oxidizing the element of a metal film, a complexing gas to form an organometallic complex by a reaction with the oxide, and a rare gas. In such a processing apparatus, the gas cluster beams collide on the metal film to etch the metal film formed on a surface of the workpiece.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 

What is claimed is:
 1. A method for processing a metal film, comprising: adiabatically expanding a mixed gas comprising an oxidation gas, a complexing gas and a rare gas in a processing chamber having a vacuum exhaust device such that a gas cluster beam is generated in the processing chamber; and irradiating the gas cluster beam upon a metal film formed on a surface of a workpiece in the processing chamber such that the gas cluster beam collides on the metal film comprising a metal element and the metal film is etched, wherein the mixed gas includes the oxidation gas which oxidizes the metal element and forms an oxide, and the complexing gas which reacts with the oxide and forms an organometallic complex.
 2. The method for processing a metal film according to claim 1, wherein the oxidation gas is at least one gas selected from the group consisting of O₂, H₂O and H₂O₂.
 3. The method for processing a metal film according to claim 1, wherein the complexing gas is at least one gas selected from the group consisting of acetylacetone, hexafluoroacetylacetone, trifluoroacetic acid, formic acid, acetic acid, propionic acid, butyric acid and valeric acid.
 4. The method for processing a metal film according to claim 1, wherein the irradiating of the gas cluster beam includes ionizing and accelerating the gas cluster beam.
 5. The method for processing a metal film according to claim 1, wherein the mixed gas has the oxidation gas in an amount which is set less than an amount of complexing gas.
 6. The method for processing a metal film according to claim 1, further comprising forming a patterned mask on a surface of the metal film, wherein the gas cluster beam etches the metal film through the patterned mask.
 7. The method for processing a metal film according to claim 1, wherein the metal element is a metal selected from a group of Cu, Co, Ni, Pt and Ru.
 8. A processing apparatus for etching a metal film, comprising: a processing chamber having a vacuum exhaust device; a holding device configured to hold a workpiece; a gas cluster beam generating device positioned to face the holding device and configured to generate a gas cluster beam through adiabatic expansion of a mixed gas comprising an oxidation gas which oxidizes a metal element and forms an oxide, a complexing gas which reacts with the oxide and forms an organometallic complex, and a rare gas; and a gas cluster beam irradiation device configured to irradiate the gas cluster beam onto a metal film comprising the metal element formed on a surface of the workpiece.
 9. The processing apparatus for etching a metal film according to claim 8, wherein the gas cluster beam irradiation device includes a nozzle configured to irradiate the gas cluster beam, and a skimming plate facing the nozzle and positioned between the nozzle and the workpiece, and the skimming plate has a gas skimming aperture configured to selectively pass a beam emitting in a selected direction in the gas cluster beam.
 10. The processing apparatus for etching a metal film according to claim 9, further comprising: an ionizer configured to ionize the gas cluster beam; and an acceleration electrode configured to accelerate the gas cluster beam ionized by the ionizer.
 11. The processing apparatus for etching metal film according to claim 10, further comprising a partition positioned parallel to the skimming plate between the skimming plate and the workpiece, wherein the ionizer and the accelerator are positioned in an ionization space formed by the partition and the skimming plate, and the partition has an irradiation aperture configured to irradiate onto the workipiece the gas cluster beam ionized and accelerated in the ionization space.
 12. The processing apparatus for etching a metal film according to claim 8, wherein the oxidation gas is at least one gas selected from the group consisting of O₂, H₂O and H₂O₂.
 13. The processing apparatus for etching metal film according to claim 8, wherein the complexing gas is at least one gas selected from the group consisting of acetylacetone, hexafluoroacetylacetone, trifluoroacetic acid, formic acid, acetic acid, propionic acid, butyric acid and valeric acid.
 14. The processing apparatus for etching a metal film according to claim 8, wherein the gas cluster beam generating device is configured to generate the gas cluster beam from the mixed gas in which the oxidation gas has an amount set less than an amount of complexing gas.
 15. The processing apparatus for etching metal film according to claim 8, wherein the workpiece has a patterned mask for etching formed on a surface of the metal film.
 16. The processing apparatus for etching a metal film according to claim 8, wherein the metal element is a metal selected from the group consisting of Cu, Co, Ni, Pt and Ru.
 17. The processing apparatus for etching a metal film according to claim 8, further comprising: an ionizer configured to ionize the gas cluster beam; and an acceleration electrode configured to accelerate the gas cluster beam ionized by the ionizer. 